Despite the central importance of neural circuit development to brain function and behavior, we lack the genetic information required to assemble a complete circuit. To address this knowledge gap we propose to develop novel synaptic and neuronal-neighborhood techniques to label a complete circuit in live animals. Using these tools we will genetically dissect a complete neural circuit for the first time, providing fundamental genetic insight into how circuits are built. We will take advantage of C. elegans biology - its available ultrastructural connectome and facile genetics, to address questions of synaptic wiring throughout a complete circuit. We will leverage the extensive genetic and anatomical knowledge of the C. elegans pharynx, a small but functionally independent circuit, as a model. We will for the first time create an expression map of all neurons in a circuit using neuron-specific FACS and transcriptomic profiling. We will then correlate these new gene expression maps to the ultrastructural connectome to reveal how patterns of gene expression correlate with connectivity. Next, we will construct fluorescent reporter systems to (1) label the entire pharyngeal circuit using GFP Reconstitution Across Synaptic Partners (GRASP) and (2) label the neighborhoods of adjacent neuronal processes using CD4-based in vivo Biotin Labeling of INtercellular Contacts (iBLINC). Using these tools, we will perform a genome-wide RNAi screen of cell surface proteins to determine their role in all major steps of circuit formation ? neuronal outgrowth, neighborhood, and synaptic choice. The results from this proposal will identify and validate genetic factors controlling wiring that can be further evaluated in complex vertebrate nervous systems and humans. This proposal will not only improve methodologies for fluorescent labeling of synapses and neuronal neighborhoods, but will provide an exquisitely detailed map-based understanding of a circuit in live animals. Furthermore, our approach and results will be useful in the broad context of understanding fundamentals of circuit formation applicable across brain complexity.